NEW YORK — Removable floodwalls would be erected in lower Manhattan, and levees, gates and other defenses would be built elsewhere around the city under a nearly $20 billion plan Mayor Michael Bloomberg proposed Tuesday to protect New York from storms and the effects of global warming.

The plan — which would also include the building of marshes and the flood-proofing of homes and hospitals — is one of the biggest, most sweeping projects ever proposed for defending a major U.S. city from the rising seas and severe weather that climate change is expected to bring.